This invention relates to a frequency discriminating circuit, in more particular, to a circuit for discriminating the frequency of an input signal or tone signal in a digital manner.
The last few years have experienced a remarkable development of techniques for processing information and the fruits of the development have come to be enjoyed by society. This has greatly affected the telecommunication technology and has led to a plan for a large-scale car-telephone network.
Such a car-telephone network is constituted in the following manner. A radio local zone is provided with a radio base station which allotted voice channels corresponding in number to the subscribers in the radio local zone. A control zone, constituted by a plurality of such radio local zones, is provided with a radio circuit control station which controls the radio base stations belonging to the control zone to establish radio or wireless circuits between the radio base stations and mobile stations (e.g. car-mounted telephone sets). A car telephone exchange station is provided for a plurality of such radio circuit control stations so as to serve to connect the mobile telephone circuits with ordinary telephone circuits and to charge the subscribers therefor.
In a mobile wireless service such as a the radio equipment mounted on a taxicab, human voice is directly used to call for a desired person so that signal control is very easy. In the case of car telephone system, however, many subscribers share a single channel with one another and moreover the connection of one channel with another is established automatically. Therefore, radio channels must be used to control originating and terminating calls.
In conventional mobile radio communication systems, car telephone systems, or the like, whenever any speech circuit has been established, a tone of a predetermined frequency within or outside the frequency band of speech voice is sent out of the base station so as to ascertain the establishment of the speech circuit desired while the mobile station receives the tone and sends back another tone indicative of the reception of the tone to the base station. Upon reception of the tone sent back from the mobile station by the base station, the completion of the establishment of the speech circuit is ascertained. The most important matter in this ascertaining operation is the frequency discriminating power of the mobile and base stations. If one or both of the stations have a poor power for discriminating frequencies, it will be impossible to ascertain the establishment of any speech circuit even if it is completed.
The necessity of discriminating the frequency of an input signal is sometimes encountered in situations other than the above described case, and the frequency discriminating means now in wide use for such situations is a filter constructed in an analog fashion. However, although such a filter can satisfactorily discriminate the frequency of an input signal in the case where input signals have all the same frequency or they have more than one frequency of which one is different from another by more than a certain sufficiently large value, it is difficult to discriminate between two adjacent frequencies in the case where the input signals have two or more frequencies and the difference between the adjacent frequencies is slight. To enable the filter to discriminate frequencies even in the latter case, the filter must have a complicated structure and therefore be expensive. Hence, there is a reasonable limit to the frequency discrimination by the use of an analog filter.